1. Field of the Invention
This invention pertains generally to voltage source inverters and, more particularly, to voltage source inverters such as, for example, medium voltage adjustable frequency drives or adjustable speed drives. The invention also pertains to medium voltage pre-charge circuits for such voltage source inverters.
2. Background Information
A voltage source inverter, such as a medium voltage adjustable frequency drive, powers a motor, such as an induction or synchronous motor, or a generator, with a suitable medium voltage.
Another example of a voltage source inverter is a variable frequency drive (VFD), which controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A VFD is a specific type of adjustable speed drive. VFDs are also known as adjustable frequency drives (AFDs), variable speed drives (VSDs), AC drives, microdrives or inverter drives. Since the voltage is varied along with the frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
As shown in FIG. 1, the main components of a voltage source inverter, such as the example medium voltage adjustable frequency drive 2, include a number of input isolation transformers 4 (two example transformers 4 are shown), a converter, such as a number of rectifier bridge assemblies 6 (two example bridge assemblies 6 are shown), a direct current (DC) bus 8, associated DC bus capacitors 10,12, and an inverter 14. The DC bus capacitors 10,12 store energy and are the voltage source for the inverter 14. As is well known, an inverter, such as 14, is an electronic circuit that converts DC to AC. An inverter performs the opposite function of a rectifier and converts a DC voltage into a variable voltage, variable frequency AC voltage.
In order to turn on the example medium voltage adjustable frequency drive 2, the DC bus capacitors 10,12 must first be charged. This process is called “pre-charge”. Without pre-charge, the inrush current to the medium voltage adjustable frequency drive 2 is relatively very large, may damage the number of rectifier bridge assemblies 6, and also may cause upstream protective relays (not shown) to operate and trip main circuit breakers (not shown).
In order to reduce the inrush current when energizing the number of input isolation transformers 4 and the number of rectifier bridge assemblies 6 that power the medium voltage adjustable frequency drive 2, the DC bus capacitors 10,12 are pre-charged. As shown in FIG. 1, a known proposal for pre-charging uses reactors (or resistors) 16 in series with the input isolation transformers 4 to limit the inrush current. Alternatively, resistor(s) can be placed in series with the output of the transformers 4, or inductor(s) can be placed in series with the transformers 4 to limit inrush. When the DC bus capacitors 10,12 are sufficiently charged, the reactors (or resistors) 16 are removed from the circuit by a medium voltage rated contactor 18.
A disadvantage of such known methods of reducing inrush current is the relatively large size and cost of the reactors (or resistors) 16 and the medium voltage rated contactor 18.
There is room for improvement in pre-charge circuits for the direct current bus of voltage source inverters.
There is also room for improvement in voltage source inverters including a pre-charge circuit.